STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net Income (Loss)	$ 24,816	$ (6,604)
Net unrealized gains (losses) on available-for-sale securities
Net unrealized holding gains (losses) arising during the period	5,996	45,202
Reclassification adjustment for (gains) losses included in net income (loss)	532	(1,800)
Total net unrealized gains (losses) on available-for-sale securities	6,528	43,402
Net unrealized gains (losses) on cash flow hedges
Net unrealized gains (losses) on cash flow hedges arising during the period	161	(1,150)
Reclassification adjustment for (gains) losses included in net income (loss)	247	(197)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax, Total	408	(1,347)
Net unrealized other-than-temporary impairments on securities
Change in previously recognized unrealized other-than-temporary impairments	(77)	(882)
Reclassification adjustment for other-than-temporary impairments included in net income (loss)		3,669
Net unrealized other-than-temporary impairments on securities	(77)	2,787
Adjustments
Value of business acquired	(871)	(5,098)
Other Comprehensive Income, Adjustments	(871)	(5,098)
Total other comprehensive income (loss), net of taxes	5,988	39,744
Comprehensive Income (Loss)	$ 30,804	$ 33,140